Just as radiative forcing provides a simplified means of comparing the various
factors that are believed to influence the climate system to one another, Global
Warming Potentials (GWPs) are one type of simplified index based upon radiative
properties that can be used to estimate the potential future impacts of emissions
of different gases upon the climate system in a relative sense. The formulation
of GWPs, reasons for the choice of various time horizons, and the effects of
clouds, scenarios, and many other factors upon GWP values were discussed in
detail in IPCC (1994). That discussion will not be repeated here. Section
6.2 discusses the relationship between radiative forcing and climate response
and describes recent studies that have supported the view that many different
kinds of forcing agents (e.g., various greenhouse gases, sulphate aerosols,
solar activity, etc.) yield similar globally averaged climate responses per
Wm-2 of forcing (albeit with different spatial patterns in some important
cases). These parallels in global mean climate responses have motivated the
use of simplified measures to estimate in an approximate fashion the relative
effects of emissions of different gases on climate. The emphasis on relative
rather than absolute effects (such as computed temperature change) avoids dependence
upon any particular model of climate response.

The impact of greenhouse gas emissions upon the atmosphere is related not only
to radiative properties, but also to the time-scale characterising the removal
of the substance from the atmosphere. Radiative properties control the absorption
of radiation per kilogram of gas present at any instant, but the lifetime (or
adjustment time, see Chapter 4) controls how long an emitted
kilogram is retained in the atmosphere and hence is able to influence the thermal
budget. The climate system responds to changes in the thermal budget on time-scales
ranging from the order of months to millennia depending upon processes within
the atmosphere, ocean, cryosphere, etc.
GWPs are a measure of the relative radiative effect of a given substance compared
to another, integrated over a chosen time horizon. The choice of the time horizon
depends in part upon whether the user wishes to emphasise shorter-term processes
(e.g., responses of cloud cover to surface temperature changes) or longer-term
phenomena (such as sea level rise) that are linked to sustained alterations
of the thermal budget (e.g., the slow transfer of heat between, for example,
the atmosphere and ocean). In addition, if the speed of potential climate change
is of greatest interest (rather than the eventual magnitude), then a focus on
shorter time horizons can be useful (IPCC, l994; Skodvin and Fuglestvedt, 1997;
Fuglestvedt et al., 2000; Smith and Wigley, 2000a,b).

As in previous reports, here we present GWPs for 20, 100, and 500 year time
horizons. The most recent GWP evaluations are those of WMO (l999) and the SAR,
and the results presented here are drawn in large part from those assessments,
with updates for those cases where significantly different new laboratory or
radiative transfer results have been published. The sources used for input variables
for the GWP calculations are indicated in this section and in the headers and
footnotes to the tables, where sources of new estimates since the SAR are identified.

The GWP has been defined as the ratio of the time-integrated radiative forcing
from the instantaneous release of 1 kg of a trace substance relative to that
of 1 kg of a reference gas (IPCC, l990):

where TH is the time horizon over which the calculation is considered, ax
is the radiative efficiency due to a unit increase in atmospheric abundance
of the substance in question (i.e., Wm-2 kg-1), [x(t)]
is the time-dependent decay in abundance of the instantaneous release of the
substance, and the corresponding quantities for the reference gas are in the
denominator. The GWP of any substance therefore expresses the integrated forcing
of a pulse (of given small mass) of that substance relative to the integrated
forcing of a pulse (of the same mass) of the reference gas over some time horizon.
The numerator of Equation 6.2 is the absolute (rather than
relative) GWP of a given substance, referred to as the AGWP. The GWPs of various
greenhouse gases can then be easily compared to determine which will cause the
greatest integrated radiative forcing over the time horizon of interest. The
direct relative radiative forcings per ppbv are derived from infrared radiative
transfer models based on laboratory measurements of the molecular properties
of each substance and considering the molecular weights. Updated information
since the SAR is presented for many gases in Section 6.3.
Many important changes in these quantities were recently reviewed in WMO (l999)
and will be briefly summarised here. In addition, some gases can indirectly
affect radiative forcing, mainly through chemical processes. For example, tropospheric
O3 provides a significant radiative forcing of the climate system,
but its production occurs indirectly, as a result of atmospheric chemistry following
emissions of precursors such as NOx, CO, and NMHCs (see Section
6.6 and Chapter 4). Indirect effects will be described
below for a number of key gases.

It is important to distinguish between the integrated relative effect of an
emitted kilogram of gas which is represented by a GWP and the actual radiative
forcings for specific gas amounts presented, for example, in Section
6.3 and in Figure 6.6. GWPs are intended for
use in studying relative rather than absolute impacts of emissions, and pertain
to specific time horizons.

The radiative efficiencies ar and ax are not necessarily constant
over time. While the absorption of infrared radiation by many greenhouse gases
varies linearly with their abundance, a few important ones display non-linear
behaviour for current and likely future abundances (e.g., CO2, CH4,
and N2O). For those gases, the relative radiative forcing will depend
upon abundance and hence upon the future scenario adopted. These issues were
discussed in detail and some sensitivities to chosen scenarios were presented
in IPCC (l994).

A key aspect of GWP calculations is the choice of the reference gas, taken
here to be CO2. In IPCC (l994), it was shown, for example, that a
particular scenario for future growth of CO2 (S650, see Chapter
3) would change the denominator of Equation 6.2 by
as much as 15% compared to a calculation employing constant pre-industrial CO2
mixing ratios.

The atmospheric response time of CO2 is subject to substantial scientific
uncertainties, due to limitations in our knowledge of key processes including
its uptake by the biosphere and ocean. When CO2 is used as the reference,
the numerical values of the GWPs of all greenhouse gases can change substantially
as research improves the understanding of the removal processes of CO2.
The removal function for CO2 used for the GWPs presented here is
based upon carbon cycle models such as those discussed in Chapter
3. The CO2 radiative efficiency (ar) used in this
report has been updated since the SAR, as discussed in Section
6.3 (see below).

The lifetimes of non-CO2 greenhouse gases are dependent largely
on atmospheric photochemistry, which controls photo-lysis and related removal
processes as discussed in Chapter 4. When the lifetime
of the gas in question is comparable to the response time of CO2
(nominally about 150 years, although it is clear that the removal of CO2
cannot be adequately described by a single, simple exponential lifetime; see
IPCC (l994) and the discussion below), the GWP is relatively insensitive to
choice of time horizon, i.e., for N2O. When the lifetime of the gas
in question differs substantially from the response time of the reference gas,
the GWP becomes sensitive to the choice of time horizon, which in turn implies
a decision regarding the climate processes and impacts of interest, as noted
above. For longer time horizons, those species that decay more rapidly than
the reference gas display decreasing GWPs, with the slope of the decay being
dependent mainly on the lifetime of the gas in question. Gases with lifetimes
much longer than that of the reference gas (e.g., C2F6) display increasing GWPs
over long time horizons (i.e., greater than a hundred years). We emphasise that
the GWP is an integral from zero to the chosen time horizon; hence the values
presented in the table for 25, 100, and 500 years are not additive.

A number of studies have suggested modified or different indices for evaluating
relative future climate effects. Here we provide only an indication of the kinds
of issues that are being considered in these alternative indices. Wuebbles and
Calm (l997) emphasised the fact that some halocarbon substitutes used, for example,
in refrigeration are less efficient than the CFCs they are replacing, so that
more energy is consumed through their use and hence more CO2 emitted
per hour of operation. Evaluation of these technological factors would only
be possible through detailed emission inventories coupled with scenarios, and
are not included here. Some studies have argued for use of “discount rates”
to reflect increasing uncertainty and changing policies with time (e.g., to
account for the possibility that new technologies will emerge to solve problems;
see for example Lashof and Ahuja, l990; Reilly et al., l999). Economic factors
could be considered along with the technological ones mentioned above, adding
another aspect to any scenario. These are not included here. Smith and Wigley
(2000a,b), Fuglestvedt et al. (2000), and Reilly et al. (l999) have examined
the relationship between GWPs and climate response using simple energy balance
models. These studies emphasised the point made above regarding the links between
choice of time horizon, lifetime of a particular substance, and the climate
response of interest, noting for example that while the 100-year GWP for N2O
represented the model-calculated temperature responses (both instantaneous and
integrated over time), and the calculated sea level rise to good accuracy, that
for CH4 (a short-lived gas with a lifetime of about 10 years) represented
sea level rise far better than it did the instantaneous temperature change for
that time horizon. For very short-lived gases, GWPs are often calculated using
a slab’ (continuous) rather than pulse emission. This approach involves
the assumption of specific scenarios for the magnitudes of the slabs. For some
very short-lived gases such as NMHCs, models must employ slab’ emissions
in GWP analyses since their impact depends critically on non-linear coupled
chemical processes. Some examples are given in Sections
6.12.3.2 and 6.12.3.4.